The present invention relates generally to program start electronic ballasts for powering discharge lamps with filament heating. More particularly, the present invention relates to program start ballasts having a resonant filament heating circuit configured with circuitry to clamp the quality (“Q”) factor of the oscillator.
Program start ballasts are known to be very useful for conditions where lights are expected to be frequently turned on and off, as they can properly operate the lamp filaments to generally extend the lamp life. To obtain a longer lamp life a program start ballast has to properly heat the lamp filaments before ignition of the lamp, but after ignition has been achieved further filament heating is unnecessary as long as the lamp current is sufficiently high.
Therefore a filament heating circuit for a program start ballast would desirably have strong filament heating capability, with a constant filament heating output voltage that is substantially insensitive to component variation and to preheat frequency.
It would be further desirable to automatically scale back or disable the filament voltage after ignition of the lamp to improve the efficiency of the total ballast.
It would be even further desirable that the ballast circuitry always work in inductive mode rather than capacitive mode to ensure soft switching during the preheat period of the half-bridge that powers the filament heating circuit. In other words, the preheat frequency should be greater than a resonant frequency for the filament heating circuit.
In any case it would be desirable to provide a filament heating circuit that is relatively simple and of low cost.
Referring to FIG. 1, a ballast 10 for powering one or more lamps 18 may be provided with a voltage driven, series resonant inverter circuit as shown that is known to those of skill in the art as an option to provide these functions. The ballast 10 may include a pair of inverter switches Q1, Q2 driven at a certain frequency (f) by a controller or drive circuit 12 which may generally be an integrated circuit 12. The switches Q1, Q2 convert an input signal from the DC voltage source Vdc into a square wave AC output. The primary winding Tp of filament heating transformer T1 and capacitor C1 in the configuration shown form a resonant tank 14. Secondary windings Ts1, Ts2 are coupled to output terminals 16 for the ballast and used to drive lamp filaments for one or more lamps that may be coupled to the output terminals 16.
Referring now to FIG. 2, an output voltage characteristic of the ballast circuit 10 of FIG. 1 is shown with respect to the switching frequency (f) of the inverter switches Q1, Q2. The output voltage Vout here is the voltage across the primary winding Tp of the filament heating transformer T1. The natural resonant frequency associated with the components Tp, C1 of the resonant tank 14 is fres. When the switching frequency (f) approaches or otherwise operates nearby the resonant frequency (fres), such as in this example at the preheat frequency (fpre), the output voltage Vout is large and output power capability is correspondingly large as well. When the switching frequency (f) operates far away from the resonant frequency (fres), such as in this example at the steady-state frequency (fsteady), the output voltage Vout will be quite small. Therefore a filament heating circuit 10 as shown is low cost, has strong preheating capability where the switching frequency (f) is near the resonant frequency (fres), and further can naturally scale back the output voltage Vout in steady state operation where the switching frequency (f) is reduced to (fsteady).
However, this circuit 10 has significant drawbacks as well. The output voltage Vout is undesirably sensitive to variations in the preheat frequency (fpre) and other component variation, as operation of the circuit at the preheat frequency (fpre) is also quite close to the natural resonant frequency (fres) for the circuit 10. Another way of describing this problem is to observe that the quality factor (Q factor) for this circuit 10 and resonant tank 14 is quite large and that small variations in frequency near the resonant frequency result in large variations in the output voltage.
Further, the operating mode of the circuit is capacitive because the preheat frequency (fpre) is less than the natural resonant frequency (fres), and therefore soft switching is not ensured.